INTRODUCTION — The term "urticarial dermatitis" describes an intensely pruritic, recalcitrant skin eruption characterized by erythematous papules and plaques that resemble urticaria but last longer than 24 hours and are sometimes accompanied by eczematous lesions [1]. Histologically, urticarial dermatitis is described by most pathologists as a "dermal hypersensitivity reaction," a nonspecific reaction pattern that is seen in a broad range of skin conditions, including drug reactions, scabies, and the prodromal phase of bullous pemphigoid [2]. However, in a substantial subgroup of patients, no underlying cause can be determined, and the diagnosis of "urticarial dermatitis" is appropriate.

EPIDEMIOLOGY — The incidence and prevalence of urticarial dermatitis are unknown. It occurs most frequently in individuals older than 50 years, with a slight female predominance [3,4].

PATHOGENESIS — The pathogenesis of urticarial dermatitis is incompletely understood. One hypothesis is that urticarial dermatitis is a lymphocyte mediated (type IV) hypersensitivity reaction. Clinical and histologic similarities have been noted between urticarial dermatitis and eruptions that occur in patients treated with anti-cytotoxic T-lymphocyte antigen 4 (CTLA-4) antibodies to break self-tolerance during immunotherapy for cancer [5]. This finding suggests that urticarial dermatitis may represent a waning of the regulatory function of the immune system that allows a reaction to develop against a self-antigen. It has been suggested that an intact basement membrane zone may distinguish urticarial dermatitis from eczematous dermatoses, and this finding may contribute to our understanding of the difference between these two entities, although additional confirmation is needed [6].

PATHOLOGY — The pathologic features of urticarial dermatitis are nonspecific and include a normal stratum corneum, mild epidermal edema with minimal spongiosis, and a superficial to mid-dermal perivascular infiltrate of lymphocytes and eosinophils with occasional neutrophils (picture 1) [2]. A few basal apoptotic keratinocytes are sometimes present. Similar features may be seen in a variety of skin conditions, including drug reactions, arthropod assault, viral infections, and prodromal stage of bullous pemphigoid.

CLINICAL MANIFESTATIONS — Patients with urticarial dermatitis typically present with an extremely pruritic, persistent eruption of dull red papules coalescing into plaques, associated with areas of urticated erythema, sometimes accompanied by eczematous lesions (picture 2C). In contrast with urticaria, lesions last for more than 24 hours and often for many days or weeks [1].

The eruption usually involves the trunk and extremities (picture 2A-B). The palms, soles, and face are typically spared. Lichenification and excoriation from rubbing and scratching is commonly seen in affected areas.

The eruption follows a chronic relapsing course. Spontaneous regression is unusual.

DIAGNOSIS AND DIFFERENTIAL DIAGNOSIS — Because the clinical and histologic features of urticarial dermatitis are nonspecific, the diagnosis is one of exclusion. A detailed history, including medication history, a complete physical examination, and additional testing (eg, complete blood cell count, direct immunofluorescence, skin scraping, patch testing), should be considered in an attempt to identify possible triggers and exclude other skin diseases that may present with similar clinical and pathologic features.

Conditions that may present as urticarial dermatitis include:

●Prodromal bullous pemphigoid – In bullous pemphigoid, the development of bullae may be preceded by a prodromal phase characterized by intense pruritus and eczematous, papular, or urticaria-like skin lesions (picture 3) that may persist for weeks to months or, infrequently, may remain as the only manifestation of the disease. The presence of any vesicles or blisters and the histologic finding of eosinophilic spongiosis and a mid- to deep-dermal infiltrate of lymphocytes and eosinophils suggest the diagnosis of bullous pemphigoid. The diagnosis can be confirmed by a skin biopsy for direct immunofluorescence and serologic test for antibodies against the basement membrane antigens BP180 and BP230. (See "Clinical features and diagnosis of bullous pemphigoid and mucous membrane pemphigoid".)

●Scabies – Scabies can mimic urticarial dermatitis clinically and histologically. Clinical findings that favor scabies include an ill-defined eruption, more significant scaling, involvement of the hands and/or feet, involvement of the male genitalia, and a history of nocturnal pruritus. On histology, scabies lesions show more spongiosis and a dense inflammatory infiltrate of lymphoid cells and histiocytes, with an admixture of eosinophils and plasma cells. However, in cases in which the two entities cannot be confidently differentiated, a positive response to empiric therapy for scabies has diagnostic value. (See "Scabies: Epidemiology, clinical features, and diagnosis".)

●Hypersensitivity drug reaction – On histologic examination, the finding of significant interface dermatitis with presence of individual necrotic keratinocytes suggests a drug reaction.